AC induction motors are widely used in drive systems of industrial vehicles such as locomotives or dump trucks. A drive system with AC induction motors geared to each axle of a bogie of the locomotive is considered to be the optimum drive system for locomotives. Similarly, AC induction motors are frequently used to drive the rear wheels of heavy mining trucks. The AC induction motors are also referred to as AC traction motors.
An alternating current, variable in both frequency and voltage, is required to effectively control the speed and power of the AC traction motors. A known method of generating such an alternating current is by the use of high-power electronic inverters using DC power from a DC link. The electrical power source for the DC link is usually rectified AC power from a diesel-powered generator set. A modern railway three-phase traction motor is controlled by supplying it with three variable-voltage, variable-frequency motor inputs from the inverter. The variations of the voltage and frequency are controlled electronically.
FIG. 1 shows the components of a traditional AC/AC locomotive drive system 10 with dynamic braking. A 3 MW diesel engine 12 is directly connected to a high-voltage three-phase alternator 14 to drive the alternator 14. The engine 12 also drives the locomotive accessories, namely air-conditioner, low-voltage alternator, air-compressor, engine and resistor grid cooling fans.
The alternator 14 generates a three-phase alternating current. The alternator 14 has a field controller 16 to generate the appropriate alternator voltage for a rectifier bank 18. The alternator 14 supplies the three-phase alternating current to the rectifier bank 18. The rectifier bank 18 converts the alternating current received from the alternator 14 to direct current.
The rectifier bank 18 charges a small capacity DC storage, referred to as the DC link 20. The DC link 20 supplies DC power to an inverter 22, which, in turn, produces a three-phase variable frequency, variable-voltage AC power for the traction motors 24. The inverter 22 operates in reverse electrical transfer mode during braking to generate power that is relayed to the resistor grid bank 26.
The traditional AC/AC locomotive drive system 10 generates AC power, which is rectified to DC power stored in a DC link, so that controlled variable voltage and frequency can be delivered to AC traction motors by an inverter of the DC power.
A control system (not shown) for the engine 12 and the alternator 14 ensures that the output power from the alternator 14 is sufficient to satisfy the power demand from the traction motors 24.
Control of the locomotive speed is obtained by monitoring the wheel speed of the locomotive and impressing a frequency demand on the inverter 22, which is between 2% and 5% greater than the wheel speed frequency. In braking, the inverter 22 produces a frequency lower than the wheel speed, the wheel torque being determined by the magnitude of the difference (slip). In order to generate the slip, a connection between the DC link (20) and the inverter (22) is needed but is not shown in FIG. 1.
During braking, all of the dynamic braking energy is dissipated in the resistor grid bank 26. The resistor grid bank 26 requires cooling, which is provided by fans driven by power from the engine 12. Thus, fuel is consumed during braking.
The traditional AC/AC drive system has a number of drawbacks, including:
a) When the vehicle is started and the frequency required by the AC traction motors is low (0.6 Hz), the distortion of the near zero frequency output of the inverter by the commutation (i.e., alternate switching each phase to positive and then to negative voltage) produces pulsations in the motor speed (referred to as “cogging”). This, in turn, leads to a lack of precision in the control of low vehicle speeds.
b) The traction power of the vehicle is controlled by the slip between the inverter frequency and the wheel speed, modulated by the square of the voltage. The magnitude of the slip applied to control the motor speed requires an accurate and rapid measurement of a small (5%) difference between two variables (sometimes 20 times larger than the slip).
c) The engine cannot act as a dynamic brake since there is no power link between the traction motors and the engine during braking.
d) Although efficient at full power, the high-current electronics nevertheless generate a substantial amount of heat, which creates a large demand for cooling power, especially in warm to hot climates.
e) The system has a large number of critical components, including a rectifier bank and inverter. Consequently, control of the system is complicated and, hence, the system may be prone to failure.
f) The constant rectification of AC current to DC current and then inverting back to AC current is not efficient. Traditional AC/AC drive systems suffer from significant power loss between the alternator and the traction motors due to the two energy-converting operations.
g) Size of components.
In order to make effective use of the torque speed characteristic of the AC traction motors throughout their entire speed range, it must be possible to obtain any desired frequency to match the speed of travel. For example, for frequencies between 20 and 120 Hz, the locus curve of the pull-out torque of a common traction motor is constant up to 40 Hz and, normally at this point, the AC/AC drive system reaches maximum power. It should be noted that for frequencies of up to 40 Hz, the supply voltage must increase in proportion to any rise in frequency.
For frequencies greater than 40 Hz, the voltage of the current supplied to the AC traction motors remains constant as it is constrained by the voltage of the DC link or by inverter electronics. For AC motors at a constant voltage, the locus curve of the pull-out torque decreases as the square of the speed. Thus, the power transferred reduces with increasing speed. The power drop-off can be reduced by use of larger motors that can produce more torque at the same voltage.
There is a need for an efficient and/or simple and/or robust temperature-tolerant AC drive system for industrial vehicles such as locomotives or heavy trucks. It would be preferable for such a system to be configured to use the engine as a sink of energy during braking and for the control system to control torque directly rather than speed.